Abstract: Penem derivatives are disclosed characterized by a substituted pyridiniomethyl group in the 2-position of the penem nucleus. The compounds of the present application exhibit better antibacterial activity than other known penem derivatives.

Abstract: A new process is described for the preparation of (5R)-penem derivatives of the general formula I: ##STR1## wherein R.sub.1 represents a hydrogen atom or an organic group; R.sub.2 represents a hydrogen atom or a carboxy protecting group and Y represents a hydrogen or halogen atom or an organic group. A 2-thiacephem derivative of the general formula II: ##STR2## wherein R.sub.1, R.sub.2 and Y have the meanings/given above is oxidized by means of organic peracids to the corresponding sulphone of the general formula III: ##STR3## which is subsequently submitted to a desulphurative ring contraction by extrusion of SO.sub.2 to give exclusively the desired (5R)-penem derivatives of the general formula I.

Abstract: A process for the preparation of compounds of formula (I) ##STR1## wherein R.sub.1 is hydrogen or an organic group;R.sub.2 is hydrogen or a carboxy protecting group; andY is a group-S-heterocyclic, acyloxy, alkoxy, alkylthio, pyridyl, which group may be substituted, or azido, and penem derivatives falling within the scope of formula (I) and pharmaceutically or veterinarily acceptable salts thereof.The process involves displacing a group L in the Y position with a reagent Y-H.

Abstract: Processes for the preparation of a penem derivative of formula I ##STR1## wherein n=0 or 1; R is a carboxy protecting group or H;R.sub.1 is hydrogen, a hydrocarbon group substituted or unsubstituted, or lower alkoxy; andR.sub.2 is hydrogen, C.sub.1 -C.sub.5 alkyl, carbamoyl N-substituted by lower alkyl or unsubstituted, or an acyl group;and the pharmaceutically acceptable salts thereof.These processes allow one to prepare stereospecifically only 5R derivatives, and are characterized by R.sub.2 -introduction at a very late stage in the synthesis, thereby enabling a great number of compounds of formula I to be prepared.Penem derivatives are useful antibacterial agents.